The present invention relates to an improvement in a silica sol-chemical grout for ground injection and to a method for accretion (i.e., strengthening the ground) by using the same.
In cases where, in constructing dams, tunnels, etc., the ground around the construction site is soft or underground water gushes forth, it has been customarily practiced to fill a grout into the soil in order to consolidate the soil or to stop the gushing of water. A chemical grout to be used for the grouting is referred to as "grouting material," and various grouting materials have already been proposed. Of known grouting materials, those in which a silica sol is used as a major component have been attracting attention since they do not cause pollution of the surrounding soil.
Grouting materials which have been most widely used are so-called water-glass grouting materials in which water glass is used as a major component together with a hardener, such as acids, salts, etc. The water-glass grouting materials can be advantageous in that they solidify within a short period of time and are capable of giving high solidification strength. However, because of their short solidification period, the gelation of such grouting materials may occur before they reach to the desired depth and, hence, the soil may not be sufficiently strengthened. In addition to this, not only sodium salts, but also acids, etc., which are used as hardeners, tend to dissolve out of the consolidated ground to pollute the surrounding soil, and the consolidated ground tends to become weak with the lapse of time.
As an improvement for such water-glass grouting materials, there have been proposed those grouting materials in which a silica sol having an extremely low sodium content is used as a major component. For example, in Japanese Patent Laid Open No. 66,482/84 is disclosed a grouting material comprising a silica sol containing a colloidal silica which has a particle size of from 6 to 50 .mu.m and contains about 0.2 to 4% by weight of Na.sub.2 O, as a major component, and a substance that releases metal ions of such metals as alkaline earth metals (e.g., calcium) and aluminum as a hardener.
In Japanese Patent Laid-Open No. 93,788/84 is disclosed a grouting material in which an alkaline silica sol is used as a major component, and a combination of (i) an acid, such as sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid, carbonic acid, etc. and (ii) a salt, such as sodium sulfate, sodium chloride, aluminum sulfate, etc., used as a hardener.
There is also disclosed in Japanese Patent Laid-Open No. 179,579/84 a grouting material in which a silica sol containing 3 to 25% by weight of colloidal silica (as SiO.sub.2) and 0.6 to 5% by weight of Na.sub.2 O is used as a major component, and a combination of (i) an organic solvent, such as butyrolactone, ethylene glycol diacetate, glyoxal, etc, (ii) an auxiliary agent, such as phosphoric acid, sodium sulfate, sodium carbonate, magnesium sulfate, magnesium carbonate, etc. and (iii) an adjuster, such as sodium hydrogencarbonate, etc. is used as a hardener.
However, the grouting material described in Japanese Patent Laid-Open No. 66,482/84 could hardly be infiltrated sufficiently into the ground because the calcium ions used as a hardener cause the gelation of the material within a very short period of time. In the case of the grouting material disclosed in Japanese Patent Laid Open No. 93,788/84, its gelation time fluctuates widely with even slight changes in the amount of aluminum sulfate added for the shortening of the gelation time. In addition, it is capable of forming a uniform gel only in the acidic region. It is therefore difficult to prepare a grouting material that exhibits a stable property. The grouting material described in Japanese Patent Laid-Open No. 179,579/84 suffers from the disadvantage that the organic solvents and the salts contained in relatively large quantities tend to dissolve out to cause pollution of the surrounding soil.